1. Field of the Invention
The present invention relates to a method for purifying contaminated industrial sewage water solutions or other aqueous solutions which have a similar contamination profile, normally comprising ions of several metals and other contaminating elements such as As, Bi, Sb, P and Se, comprising precipitating the contaminants in two stages and returning the precipitate isolated from the second stage to the first precipitation stage.
2. Description of the Related Art
Many known methods for purifying industrial sewage water are directed primarily to certain selected contaminants or impurities and are less effective in the case of other contaminants. These processes are often complicated or often contain stages which require the use of advanced and sensitive control equipment, for instance precipitation agent metering equipment or pH-regulating equipment. Because of their complexity, sensitivity and cost, such purification methods are only used to a limited extent, or are less effective than would be desired.
Methods of the aforementioned type are known from Boliden's earlier patent specifications, for instance from EP-B-O,139,622 and EP-B-0,364,423, these publications herewith being incorporated into the present description. A fundamental concept of both of these methods is that the major part of the precipitation reagent is added in the second stage under conditions in which a voluminous metal hydroxide-metal oxide precipitate is formed. This voluminous precipitate is separated and returned to the first precipitation stage and contains a substantial part of the contaminant content and is used as an auxiliary precipitation agent in the precipitation taking place in the first stage, from which there is taken a precipitate which includes essentially the entire contaminant content of the contaminated solution. According to EP-B-0,139,622, the contaminants removed are primarily As and/or P, wherein the precipitation process in the first stage is carried out in an alkaline environment to form an arsenic-containing and/or a phosphate-containing dumpable sludge. Publication EP-B-0,364,423 relates to the purification of industrial waste solutions, for instance solutions that contain As, P and also heavy metals, wherein after adding the voluminous hydroxide precipitate taken from the second stage, purification in the first stage is effected by adding sulphide ions at a pH such as to dissolve the hydroxide precipitate. This results in the formation of a metal sulphide precipitate which can be conveniently worked-up with respect to its valuable metal content.
These known two-stage processes in which hydroxide precipitate is returned from the second stage purify industrial water sewage much more effectively and in a much simpler manner than the earlier purification processes, among other things with regard to the deficiencies of the known processes mentioned in the introduction and also with regard to general problems that occur in such purification processes. However, the aforesaid known two-stage processes can be considered too sophisticated from a technical aspect and also to be encumbered with other drawbacks in certain respects and in the case of certain uses.
The problem associated with the choice of a method for purifying industrial waste effluents, sewage water or similar contaminated aqueous solutions resides in the constant increase in the demands for generally low influence on the environment placed by the responsible authorities, for instance the limit values of the effluent to the recipient are made successively lower. The number of substances or elements to which the limit values apply also has become successively larger in number. The tolerance with which industrial effluents are considered in operational breakdowns or abnormal situations is decreasing, and consequently even short stoppages in an industrial processes which result in the discharge of effluent above current limit values can have very serious consequences for those of responsibility in the industry concerned.
These new and progressively growing requirements on reliable and operationally safe purification processes accentuate still further those problems indicated above with regard to known processes for the purification of industrial water sewage. Thus, the demands for operational safety and reliability are in addition to the earlier requirements of a good precipitation result and low outgoing residual contents. The two known Boliden methods are able to well satisfy the modern demands placed on all of the aforesaid, conceivable parameters, with regard to their particular applications and with regard to the conditions stated, for instance the presence of relatively high concentrations of As.
However, in purification situations in which the solutions or the water sewage to be purified contains highly varying concentrations of contaminants and/or a broad spectrum of contaminants, there is no method which can be applied in practice and which can be used generally to achieve accepted residual contents or concentrations while providing an operationally safe and reliable alternative at the same time.